The present invention relates to a process of preparing a sprayable liquid coating composition wherein gaseous carbon dioxide is utilized to reduce the viscosity of a concentrated solution comprising an edible polymer and a solvent. The addition of the gaseous carbon dioxide in the concentrated solution permits a sprayable composition to be produced using a significantly reduced level of solvent in the edible polymer/solvent solution. Typical solvents used in the process have been deemed to be volatile organic compounds (VOCs). Thus, by reducing the amount of solvent used in the solution, the level of VOC emission during the coating process also is reduced. Either supercritical carbon dioxide or subcritical carbon dioxide can be used as the gaseous carbon dioxide in the present invention.
It is well known to coat pharmaceutical and food products with an edible polymer in order to prevent degradation of the edible product, particularly degradation by moisture and/or oxidation. A number of edible polymers have been used in the prior art including for example shellac, cellulose derivatives, terpene resins and synthetic carboxylic polymers. These edible polymers are dispersed in a carrier or solvent and applied to the edible product by various means, such as panning, spraying, brushing or curtain coating.
The use of shellac as an edible polymer coating has increased in recent years.
Shellac is a naturally occurring resin of animal origin, derived from the seedlac of the tiny scale insect Laccifer lacca. Although the precise chemical nature of shellac has yet to be determined, it is the only known commercial resin of animal origin. Shellac""s continued use as a resinous coating is due to its water resistant and lustrous finishing properties. Shellac can be manufactured by a solvent process to produce three types of shellac: dewaxed, dewaxed decolorized and wax-containing. For the wax-containing grade, raw seedlac and solvent, typically ethyl alcohol, are charged into a dissolving tank at a ratio of 1:4 by weight, refluxed and filtered. The wax content of these shellacs can be controlled using different proof alcohol to dissolve the lac. Dewaxed shellacs are made by dissolving seedlac in either (a) cooled alcohol of high proof or (b) weaker proof alcohol at slightly elevated temperature. Dewaxed decolorized shellac is produced in the same manner as the dewaxed shellacs followed by a treatment with activated carbon to remove the darker coloring material. Another type of shellac is bleached shellac which is produced from seedlac of Indian or Thailand origin. The seedlac is dissolved in an aqueous alkali solution, such as sodium carbonate, at a high temperature, and processed to remove impurities.
The versatility of shellac in coating compositions is demonstrated in its varied applications. It has been applied to wood, metal, glass fibers, foil, plastics, paper, ceramics, leather, rubber, hair, fruits, candy and tablet. In addition, shellac can be applied by any number of techniques, including brushing, rolling, doctoring, tumbling and spraying (Martin, J. W. xe2x80x9cShellacxe2x80x9d, Bradshaw-Praeger and Co. Chicago, Ill., p. 442-476). The viscosity of the shellac must be reduced in order to use it in a coating composition. Shellac generally is not water soluble, tending to form a colloidal dispersion. Thus, shellac typically is dissolved in a solvent, such as an alcohol, in order to reduce its viscosity. Current practices include dissolving from about 5 to 10 wt. % (Merl, J. A. and Stock, K. W., xe2x80x9cSilesia Confiserie Manual No. 4xe2x80x9d, Silisia Gerhard Hanke KG, Abt., Neuss Germany, 1996, p. 84) to up to about 45 wt. % (Martin, J. W. xe2x80x9cShellacxe2x80x9d, Bradshaw-Praeger and Co. Chicago, Ill., p. 466-470) shellac in the solvent. More commonly, about 30 wt. % of shellac is dissolved in the solvent (Mitchell, N. E. xe2x80x9cThe Clean Air Act Its Effect on Panning Candiesxe2x80x9d, Manufacturing Confectioner, October 1999, p. 41-44). Edible film coating compositions comprising an edible shellac dissolved in an alcohol-based solvent are described in U.S. Pat. No. 4,661,359 to Seaborne et al, issued Mar. 7, 1989, U.S. Pat. No. 4,710,228 to Seaborne, issued Oct. 16, 1985 and U.S. Pat. No. 4,810,534, issued Mar. 7, 1989.
One conventional process for coating edible products is panning. Panning involves tumbling the edible product (such as tablets, candies, etc.) in a revolving drum. As the product is tumbled, the edible shellac/alcohol solution is sprayed or ladled into the drum. Drying air is introduced to the pan in order to evaporate the alcohol, and the alcohol is exhausted into the air handling system and out of the factory. An example of such a process is disclosed in U.S. Pat. No. 3,949,096 to Johnson et al., issued Apr. 6, 1976, which describes an edible surface coating dispersion comprising an edible coating material and a fugitive solvent, wherein the solvent is volatized in a heating zone to leave a dry surface coating.
Ethyl and isopropyl alcohol are classified as a volatile organic compounds (VOCs) or volatile organic materials (VOMs). Volatile organic compounds are one cause of pollution, mostly in the form of ground level ozone, which is a highly reactive gas that can be harmful to the public and contribute to smog. Consequently, a serious drawback to the use of the shellac/alcohol coating solution is the emission of volatile organic compounds (VOCs). For example, a 55-gallon drum (about 400 pounds), of which 70% is ethanol, yields about 280 pounds of VOC fugitive emissions (Giesecke, A., xe2x80x9cVolatile Organic Compounds (VOCs) xe2x80x9d, Manufacturing Confectioner, October 1998, p. 77-78).
The Environmental Protection Agency (EPA) has designated certain areas as xe2x80x9cnon-attainment areasxe2x80x9d in order to regulate the amount of permissible VOC production for a given facility. There are several types of xe2x80x9cnon-attainment areasxe2x80x9d including moderate, serious, severe and extreme; different rules regarding the level of permissible VOC emissions have been imposed for each area. For example, Chicago is classified as a severe non-attainment area. Chicago-area confectionary companies produce a significant portion of all panned candies sold in the United States. The process of pan-polishing of candies generally utilizes ethyl alcohol as the main solvent in the glaze, the ethyl alcohol being emitted into the atmosphere as a VOC in the absence of any controls. EPA restrictions now limit the VOC content of a glazing mixture to 3.5 lbs/gallon. If this limit cannot be achieved, control of at least 81% of the overall VOC emissions must be established. However, the currently available glazing systems and non-compliant glazing mixtures generate more than 5 lbs of potential VOCs/gallon (Mitchell, N. E. xe2x80x9cThe Clean Air Act Its Effect on Panning Candiesxe2x80x9d, Manufacturing Confectioner, October 1999, p. 41-44).
Thus, in panning techniques where the solvent levels are too high, systems for capturing the solvent must be utilized. However, conventional panning emissions are fugitive VOC emissions that are not readily addressed by typical stack controls such as catalytic or thermal oxidizers or other available VOC reduction technologies (Potter, C., xe2x80x9cVOC Emission Limitations from Candy Manufacturing Facilities in Californiaxe2x80x9d, Memorandum to Stephanie Smith, National Confectioner""s Association, Mar. 22, 2000). Therefore, xe2x80x9cscrubbingxe2x80x9d the exhaust stream in order to contain the VOCs is required, but it also is very cost prohibitive. As a result, there have been attempts to replace the alcohol with a substitute solvent, such as water or acetone. While acetone is legal in all states except California, the volatility of acetone as a substitute solvent makes it dangerous for use in panning operations. Water-soluble glazes also have been used as a substitute coating solution, but the production time is increased substantially due to increased drying time. In addition, the water-borne coatings are often susceptible to problems with humidity, such that the products begin to stick together.
Carbon dioxide specifically is excluded from the definition of VOCs (Stevens, J. P., xe2x80x9cAssessment and Abatement of Volatile Organic Materialsxe2x80x9d, Manufacturing Confectioner, November 1999, p. 60-66). Thus, the present invention contemplates the use of carbon dioxide as a viscosity reduction agent for an edible polymer suitable for use in a coating composition, thereby reducing or eliminating the required amount of solvent. U.S. Pat. No. 4,923,720 to Lee et al., issued May 8, 1990, provides a process and apparatus for using supercritical fluids, such as carbon dioxide, to reduce to application consistency viscous coatings in liquid spray applications. The Lee et al. patent generally discloses a process for a liquid spray application for coating a substrate comprising forming a liquid mixture from at least one polymeric compound capable of forming a coating on a substrate, at least one supercritical fluid, such as supercritical carbon dioxide, and optionally an active solvent, and then spraying the liquid mixture onto a substrate in the form of droplets having an average diameter of 1 micron or greater. The described process is cumbersome, including the requirement that the final liquid mixture be heated to avoid condensation of carbon dioxide and ambient water vapor prior to its introduction to the spray nozzle. The Lee et al. patent does not address using a supercritical fluid in the formation of edible coatings, nor does Lee et al. specifically provide for a reduction of volatile organic compounds (VOC) emissions. In addition, Lee et al. teaches away from the use of subcritical fluids, such as subcritical CO2 in its liquid spray application.
Despite the advances of the prior art, a need still exists for a process and apparatus for coating foods with an edible polymer using gaseous carbon dioxide. Such a process and apparatus should lower significantly the amount of VOC emissions resulting from panning techniques of edible shellac solutions as imposed by the Clean Air Act of 1990. Such a process and apparatus also should provide an edible shellac solution that can be sprayed while using significantly lower amounts of VOCs. In addition, such as a process and apparatus should yield equal, if not improved, production rates, relative to processes and apparatus currently being used.
Accordingly, it is an object of the present invention to provide an edible sprayable liquid coating comprising a concentrated solution of an edible polymer and a solvent which is mixed with gaseous carbon dioxide.
It is also an object of the present invention to provide an edible sprayable liquid coating comprising a concentrated solution of edible shellac and an alcohol which is mixed with gaseous carbon dioxide.
It is another object of the present invention to provide an edible sprayable liquid coating which utilizes supercritical carbon dioxide or subcritical carbon dioxide in order to reduce the viscosity of the concentrated solution of edible polymer and solvent.
It is yet another object of the present invention to provide an edible sprayable liquid coating which can be applied onto confectionary and pharmaceutical tablets.
It is an additional object of the present invention to provide an edible sprayable liquid coating having a reduced amount of solvent in order to reduce volatile organic compound (VOC) emissions.
It is a further object of the present invention to provide a method of applying a sprayable liquid coating onto an edible substrate which reduces the amount of volatile organic compound (VOC) emissions.
It is still another object of the present invention to provide a method of applying a sprayable liquid coating onto an edible substrate which increases production rate.
It is an additional object of the present invention to provide a method of applying a sprayable liquid coating onto an edible substrate which reduces the risk of explosion.
It is another object of the present invention to provide an apparatus for producing a sprayable liquid coating from a concentrated solution of edible polymer and solvent which utilizes supercritical carbon dioxide.
It is also an object of the present invention to provide an apparatus for producing a sprayable liquid coating from a concentrated solution of edible polymer and solvent which utilizes subcritical carbon dioxide.
Additional objects, advantages and novel features of the invention will be set forth in part by the description and claims which follow, and in part will become apparent to those skilled in the art upon examination of the following specification or may be learned by practice of the invention.
These and other objects of the present invention are accomplished by providing a process of preparing a sprayable liquid coating wherein gaseous carbon dioxide is utilized to reduce the viscosity of a concentrated solution of an edible polymer and a solvent. In this manner, a sprayable composition is produced using a significantly reduced level of solvent in the solution, thereby significantly reducing the level of VOCs emissions during the coating process.